1. Technical Field
The disclosed embodiments relate primarily to the extraction of circuit simulator parameters and/or design rules from test structures, and more particularly from ring oscillator test structures.
2. Background Information
Circuit simulation tools are commonly used in the design of integrated circuits. One commonly used circuit simulator is a program called SPICE (Simulation Program with Integrated Circuit Emphasis). A designer of a circuit to be realized in integrated circuit form describes the circuit as a netlist. The netlist defines an interconnected set of circuit components. Some examples of circuit components are basic circuit elements such as resistors, capacitors, inductors, voltage sources, and current sources. Operation of these circuit elements are usually a function of one or a small number of parameters. For example, a basic resistor is defined by a single parameter, a resistance. The netlist typically, however, includes more complex circuit components such as lengths of metal interconnect, an N-channel field effect transistor and a P-channel field effect transistor. Operation of a more complex circuit component is typically defined by a “model” involving one or more equations. The model defines operation of the circuit component as a function of multiple parameters. For example, a model of a length of interconnect may use parameters such as the running length of the interconnect, the cross-sectional width of the interconnect, the cross-sectional thickness, the distances between the interconnect and other structures, and the dielectric constants of other materials surrounding the interconnect, and other physical dimensions and electrical properties. There are many different ways of modeling circuit components. Some models are complex, whereas other models are relatively simple.
In the design a particular circuit to be realized in integrated circuit form, the models and the parameters the models use are carefully selected such that simulation of a circuit being designed will closely match actual operation of the circuit if the circuit were to be realized in integrated circuit form. After the circuit being designed has been described as a netlist, the SPICE program simulates the described circuit to determine its simulated operation. If the circuit as simulated does not work correctly, then the circuit design is changed. In this way, a circuit design is simulated and its proper operation is confirmed before the circuit is fabricated.
The values of the parameters are typically determined in a process sometimes referred to as “parameter extraction.” In one example of parameter extraction, a suite of test structures is fabricated on a test die using the semiconductor manufacturing process that will later be used to fabricate the circuit to be designed. The test structures are then tested. The test structures are simulated using SPICE, and the parameters of the various models are adjusted until the SPICE simulations match the test results. See, for example, U.S. Pat. Nos. 7,047,505 and 6,934,671 and 6,795,800 for examples of different parameter extraction processes.
It is common to fabricate a ring oscillator as one such a test structure, then to operate the ring oscillator, and to measure its operating frequency. Relevant SPICE circuit parameters are adjusted such that SPICE simulation of the ring oscillator outputs a simulated oscillating frequency that matches the measured oscillating frequency. Examples of parameters that might be adjusted include the length of the gates of the transistors of the ring oscillator, the width of the gates, the dielectric constant of the gate insulator of the transistors, the thickness of the gate insulator, the threshold voltage of the transistors, the mobility of carriers in the transistors, and many other parameters. See, for example, U.S. Pat. Nos. 5,790,436 and 5,825,673. Often a second ring oscillator is fabricated on the same test die such that the second ring oscillator utilizes a different type of transistor. This second ring oscillator is used to calibrate the parameters used by the model of the transistors in the second ring oscillator. Circuit operation is not only, however, affected by transistor performance. Circuit operation is also affected by the electrical characteristics of the interconnect used to connect the various active circuits together. Lengths of different types of interconnect are sometimes fabricated on the test die along with the ring oscillators. Various electrical characteristics of these lengths of interconnect are measured to extract parameters that are used in the SPICE models for interconnect. In addition, multiple ring oscillators are sometimes fabricated on the test die such that the various ring oscillators have different amounts of interconnect in the oscillating signal paths and therefore operate differently. Sometimes a single ring oscillator is tested in an unloaded condition and then is tested again in a loaded condition. See, for example, U.S. Pat. No. 6,055,829. By comparing the operation of such ring oscillators, the effect of interconnect loading on circuit performance can be isolated to some degree and can be used to change SPICE parameters used by the interconnect models. The extraction of SPICE parameters is a complicated and time-consuming multi-dimensional task involving many different interrelated parameters and different types of test structures and different types of models.